Résumé The most critical hurdle on the path to sustainable development in Africa and the rest of the so-called Third World is poverty, commonly manifested as food security. A number of factors threaten food security in Zimbabwe, and these include climate change, an unstable socio-political environment and economic depression. The major debates and initiatives on sustainable development often fail to focus on the eradication of poverty in southern Africa. As a result, the trade liberalisation programmes signed by African countries in economic partnership agreements leave smallholder farmers vulnerable to the influx of hybrid seed and genetically modified organisms (GMOs), which threaten local agro-biodiversity. This case study has shown that farmers select seed to plant for food as a result of various learning interactions they engage in, which include inter-generational knowledge transfer, farmer to farmer extension and external training by extension organisations and NGOs. A communities of practice (COPs) (Wenger, 2007) framework was used to gain an understanding of the learning interactions among farmers and their stakeholders in Nyanga and Marange COPs of small grain farmers in Manicaland Province, Zimbabwe, who have been working with Environment Africa (A regional NGO). A critical realist causal analysis was conducted to unravel the various causal factors influencing choice. A number of underlying structures and causal mechanisms were found to influence learning interactions and choices in these communities of practice, and they include ambivalence, which influences the changing domain and practice. Climate change, drought and risk were found to affect farmer practice, while power relations affect the community, its practice, domain, sponsorship and the learning interactions in the COPs. The political economy was also found to have a profound effect on the domain and practice. A space was found for the influence of capacity and knowledge sharing in participatory frameworks of the communities, implying that extension quality can be enhanced to promote locally adapted and diverse seed varieties for food security improvement. The study shows that a deeper understanding of the mechanisms influencing the context of teaching and learning provides a more refined insight into the learning interactions and choices of farmers. This, coupled with the social processes descriptors provided by Wildemeersch (2007) has given me a more detailed understanding of the nature of learning interactions influencing farmer choices.